The present invention relates to a backlash clutch that realizes a high torque transmission characteristic and free joint mechanism. This invention also relates to a free joint mechanism for a robot comprising the clutch.
Generally, in a clutch mechanism, connection and disconnection for transmitting driving force are achieved by means of a friction mechanism. Such a friction mechanism comprises rotating members that are joined together by frictional function acts between the members to control the on-off action of the clutch. In such a clutch mechanism, using frictional action, it has been difficult to transmit a force when the force is large because slippage arises as the clutch mechanism transmits a driving force.
This phenomenon generates a significant problem when a clutch mechanism is applied to a knee joint of a humanoid robot. Typically, in a humanoid robot, such a joint mechanism provided in the body is driven by controlling actuators to generate objective paths. In particular, an actuator is used to generate high power to the knee joint.
When the humanoid robot rests with a posture shown in FIG. 9, assuming that the weight of the upper part from the articulatio coxae is 60 kgf and the offset length from the articulatio coxae to the knee joint is 10 cm, and that the acceleration of gravity is 9.81 m/s2, the actuator that drives the knee joint must output a high torque, such as 58.86 Nm. Therefore, the knee joint mechanism must have a capability to transmit such a high torque.
In general, the knee joint mechanism is composed of a motor generating high power and reduction gears having a high gear ratio. On the other hand, in a human's walking motion, the knee joint of a leg acts as a free joint when the leg is not engaging the ground (grounded or grounding). It is contemplated that such an action of the knee joint has the following effects;                There is no need to use extra force.        When a human moves a leg forward, the knee of the leg moves freely in accordance with inertial force.        When the leg touches down, an impulsive reaction force that acts on the leg eases up as the leg heads for ground at an adequate angle, naturally.        
However, a conventional knee joint mechanism has difficulty realizing a free joint characteristic following a reactive force, where control of the walking motion is performed so as to maintain the knee with a desired angle when the leg is not grounded.
To ensure that the knee joint has such a free moving condition, it is contemplated to incorporate a clutch mechanism into the reduction gears. However, a general clutch mechanism is a friction-type clutch, such as the above-mentioned clutch, and it slips at high torque, such that it is difficult to transmit high torque. Therefore, the clutch mechanism can only be attached to the input shaft side of the reduction gears. Consequently, the need exists for a clutch mechanism for a knee joint of a humanoid robot that can provide output at the shaft side of the reduction gears, as a mechanism that can both transmit high torque and maintain a free moving condition.
It is therefore an object of the present invention to provide a clutch mechanism capable of transmitting high torque and maintaining a free moving condition, and a joint mechanism of a robot comprising such a clutch mechanism. In particular, an object of this clutch mechanism is to realize a walking motion that has the following advantages when it is used as a knee joint:    (1) Transmits a large driving force.    (2) Achieves a free joint.            (a) Does not need to use extra force.        (b) Realizes free motion.        (c) Absorbs an impulsive/reaction force at landing (grounding).        